1. Field of Invention
The present invention relates to a new type of stator design and arc shaping method for brushless motors. More particularly, the present invention relates to a stator design and arc shaping method for diametrically wound and diametrical air gap type of brushless motors.
2. Description of Related Art
Since brushless motors have properties that allow them to be miniaturized and flattened, they are now extensively used inside small and high precision automatic machines. At present, permanent magnet brushless motors can be classified into three main types, namely: a diametrically wound and diametrical air gap type, a diametrically wound and axial air gap type, and an axially wound and axial air gap type. Most high speed CD-ROMs use three-phase diametrically wound and diametrical air gap type of brushless motors. It does not matter if the motor has an 8-poles/9-grooves or a 12-poles/9- grooves configuration, the stator always contains a number of radiating teeth having grooves formed in each. Coils are wound round the belly of the teeth. To increase the efficiency of a diametrically wound and diametrical air gap type of brushless motor, saturated magnets are often used in substitution for ordinary permanent magnets. Due to a magnetic cross-linking between the permanent magnets and the stator, the so-called cogging torque is produced in the motor. When saturated magnets are used inside the motor, cogging torque will be increased. A larger cogging torque can adversely affect the operation of the motor, leading to vibrations, noises, a reduced motor life and operational instability. If motor having an oversized cogging torque is used, for example, in a CD-ROM or a hard disk, there will be substantial rotational speed variation. This may result in poor data reading and writing quality.
Conventionally, there are a number of patents directed towards the cogging torque problem of brushless motors. For example, U.S. Pat. Nos. 4,769,567, 4,847,712, 5,013,953, and 5,030,864 are all attempts to reduce cogging torque. To reduce cogging torque, one must analyze the motor structure, and improves either the stator design or the rotor design. U.S. Pat. No. 5,030,864, for example, seeks to change the groove opening size of a stator to obtain a lower cogging torque. Although the reduction of cogging torque is achieved as stated, a larger groove opening means a lessening of winding space available. Besides, the reduction of magnetic flux into the stator will lower the overall performance of a motor. In U.S. Pat. No. 5,250,867, holes are punched in the periphery of the silicon steel sheet; and in U.S. Pat. No. 4,672,253, rectangular grooves are ground in the silicon steel sheet to lower the cogging torque. However, when the above two methods are applied to the fabrication of a small motor such as the one used in a CD-ROM, the techniques are too complicated.
In light of the foregoing, there is a need in the art to provide an improved stator design.